FLAMEHOLDER

Abstract: The invention refers to a flameholder device, particularly for ram-jet engines and after-burners in turbojet engines and comprising an annular body (2) coaxial with the centre axis of the engine and formed by a plurality of circumferentially spaced thin-walled metal guide vanes (4), upstream of which fuel nozzles (1) are mounted and downstream of which a turbulent combustion-stabilizing zone is formed, said guide vanes (4) being hollow and having relatively great volume and which, if desired, are cooled by means of air flowing therethrough, the guide vanes furthermore being protected by a ceramic material (3). In order to prevent the intrusion of the fuel-air mixture into the internal space of said guide vanes should cracks occur in said vanes and hence accompanying destructive burning thereof, it is suggested according to the invention that the ceramic material is made as a closed box-like insert body (3) arranged within the cavity in the guide vanes and with its external surface all around lying close to the internal surface of the guide vane walls and which either is hollow in itself and in the case of cooling by means of a flow of air therethrough have openings corresponding to the through-flow openings in the guide vane walls or is filled with a porous ceramic material or entirely consists of such porous material.

Description: Flameholder The present invention refers to a flameholder device, particularly for ram-jet engines and after-burners in turbo¬ jet engines and comprising an annular body coaxial with the centre axis of the engine and being in the shape of a plura¬ lity of circumferentially spaced thin-walled metal guide vanes, upstream of which fuel nozzles are mounted and down¬ stream of which a turbulent combustion-stabilizing zone is formed, said guide vanes being hollow and having relatively great volume, and said guide vanes, if desired, being cooled by means of air flowing therethrough, said guide vanes fur¬ thermore being protected by a ceramic material. The function of flameholders is to create turbulence and wakes in the flow particularly in after-burners of jet engines but also in other kinds of engines such as ram-jet engines for space propulsion. In the turbulent zone the mix¬ ture of fuel and air has time to be lit before it flows out of the engine. In this way the combustion is kept within the engine and provides an additional thrust. A flameholder device is subjected to high gas flows, high temperatures and rapid temperature changes. The heat radiation from the flame directly downstream provides very high surface temperatures. From the core engine gases leave the turbine exit which have a temperature of about 900°C. Furthermore, cold fuel is supplied through fuel nozzles directly upstream the flamehol¬ der which leads to thermo-chock effects. Today flame-holders are made of metal material having a thermally isolating surface layer of Zr0 2 . Decisive of the service life of the flameholder is high temperature corrosion and oxidation combined with thermo-mechanical fatigue. In order to increase the service life of the flameholder it may be cooled with the aid of the fan air of the motor. This is described in WO 92/21872. Flameholders made entirely of ceramic composite material also have been suggested in order to obtain a still better or improved service life and have been tested but uncertain material data and the brittleness of ceramic materials may give, however, inferior reliability of said type of flame holder. The flame holder might be made from annular bodies having V-formed crossectional shape or guide vanes which guide the flow into a spiral motion. As far as guide vanes are concerned, they often have a great internal volume which may be closed or passed by a flow of cooling turbine outlet gases. Should cracks occur at the front edge of the guide vane due to thermo-chock from the cooling liquid fuel, the fuel-air mixture can flow into the guide vane and start to burn within the internal volume thereof. This leads to a fast destructive burning of said guide vane such that the flame- holding function no longer might be assured. From US-A-2,964,907 is known a flameholder device for an after-burner to a jet engine, said flameholder having a V- shaped cross section and circumferentially spaced bodies 29 of ceramic material mounted for cooling purposes in the mouth of the V-profile. From US-A-5,090,918 is also known a design having guide vanes which are made entirely of ceramic mate¬ rial. Finally, from CH-A-559,882 it is also known a combustor wall design comprising a filling of porous ceramic material. The main object of the present invention therefore is to suggest a flameholder device in which the drawbacks of prior constructions are eliminated. According to the inven¬ tion, this is now achieved substantially in that the ceramic material is made as a closed box-like insert body within the cavity of the guide vanes, which insert body with its outer surface lies close to the internal surface of the guide vane walls at all places and being hollow itself or, in case of cooling by an air flow, having opening corresponding to the through-flow openings in the guide vane walls, or is filled with a porous ceramic material or entirely consists of such porous material. Owing to the invention it can now be effec- tively prevented that in case of cracks occuring in the walls of the metal guide vanes this can lead to the intrusion and combustion of fuel-air mixture at the inside of said walls. By way of example, the invention will be further described below with reference to the accompanying drawings, in which Fig. 1 is a side elevational view, partly in cross section, of a jet engine with an inventive after-burner flame holder device, Fig. 2 illustrates in an enlarged view a portion of the sectioned part of the very flame-holder of the engine of Fig. 1, Fig. 3 illustrates in section and in an end view inventive guide vanes, Fig. 4 is a perspective view of a flameholder device comprising guide vanes of Fig. 3 accor¬ ding to the invention, Fig. 5 illustrates various possible combinations of metal and ceramic materials according to the invention and Fig. 6 and 7 is a longitudinal and, a cross- sectional view, respectively, of an air-cooled embodiment of« an inventive guide vane. In Fig. 1 is illustrated a turbojet engine in a partly sectioned side elevational view to show the after- burner portion located therein. In said portion is concentri¬ cally to the centre axis of the engine mounted an annular flameholder 2 with V-shaped cross section. Upstream the flameholder 2 is located a pilot nozzle 1 which belongs to the after-burner portion and in the annular body of the flameholder are mounted a number of circumferentially dis¬ tributed thin-walled metal guide vanes 4. According to the invention there is arranged in the internal cavity of the guide vanes a closed box-like insert body 3 of fibre-reinfor¬ ced ceramic material. With its external surface said body follows essentially the contour of the limiting walls of the internal cavity and can itself be hollow. From Figs. 3 and 4 is more clearly evident how the flameholder comprises a number of circumferentially distribu¬ ted guid vanes 4. As most clearly is evident from the end view in Fig. 3 said guide vanes have an enlarged airfoil¬ like shape and therefore their internal volume becomes great and according to the invention said volume thus houses a fibre-reinforced ceramic body 3 with a peripheral contour which at all places lies close to the inner surface of the guide vane walls. The above-stated fibre-reinforced ceramic bodies now provide for an essentially reduced risk for destructive burning of the flameholder portions and allow the use of an already known and existing flameholder structure of sheet metal. In such case the fibre-reinforced ceramic body is entirely embedded into the metal construction and does not need to serve as load-carrying structural element. Therefore, the device according to the invention is very reliable. Ceramic materials which are possible to use in this connection are firstly ceramic composites with continuous fibres of SiC, Si 3 N 4 , A1 2 0 3 or mullite (or other carbides, nitrides, oxides or borides) in a ceramic matrix of the same group of materials. As example of possible ceramic composi¬ tes it is to be mentioned SiC f /Si 3 N 4 , SiC f /Al 2 0 3 and SiC f / SiC, where the index f means fibre. Also monolitic ceramics, i.e. without fibre reinforcing, may be used. An example thereof has been illustrated in Fig. 5a which shows a ceramic insert 3 for a flameholder guide vane 4. A further possible solution is illustrated in Fig. 5b, in which a porous vacuum-formed fibre-reinforced body 3, typically having about 80% porosity, entirely fills the internal volume as illustrated in Fig. 2. Said fibre body 3 may consist of e.g. fibres of A1 2 0 3 , Si0 2 , mullite, Zr0 2 or aluminium silicate glass (or another oxide, carbide, nitride or boride) of loose-wool type, which is vacuum-formed with an inorganic binder which may be a Si0 2 -sol. This type of mate- rial is previously known in connection with the use as ther¬ mal isolation in ovens and in space vessels. It is also possible to combine a ceramic insert 3 with a porous fibre-reinforced body. The ceramic insert then supports that surfaces of the guide vane 4 which are most loaded, see Fig. 3 and 5c. In the invention the form stability of ceramic mate¬ rials at high temperatures is utilized without having to rely upon said ceramic material as a load-carrying component in normal conditions of operation. In Figs. 6 and 7 has been illustrated an embodiment of the guide vanes 4 which is cooled by hot air flowing therethrough. In this case the insert body 3 is hollow and provided with suitable inlet and outlet openings for said airflow in accordance with the corresponding openings in the surrounding guide vane walls at an upstream inlet ladle 9 and a downstream flame zone 10. The following examples of application tests with the invention may be mentioned. Example 1 A ceramic insert having the shape according to Fig. 3 was made of a ceramic composite material. The material was SiC-fibre/Al 2 0 3 -matrix manufactured by the Dimox®-method by DuPont Lanxide Composites, Inc., USA. Said insert was located within a flameholder guide vane such as illustrated in Figs. 3 and 4. The guide vane was welded and soldered in the same way as when manufactured without ceramic insert. Said metal casing was entirely without openings, i.e. non-cooled. Also the ceramic insert was made without openings or holes. The flame holder was mounted in the after burner portion of the turbojet engine. The results of the engine test showed an essentially increased service life and form stability of said flameholder guide vane made according to the invention. Example 2 A ceramic insert with a shape according to Fig. 3 was made of a ceramic composite material. The material was SiC-fibre/Al 2 0 3 -matrix made by the Dimox®-method by DuPont Lanxide Composites, Inc., USA. The insert was located within a flameholder guide vane such as illustrated in Fig. 3. The guide vane was welded and soldered in the same way as when made without ceramic insert. The metallic casing was cooled by letting turbine exhaust gases flow through holes in the casing. The flame holder was mounted in the after-burner of a turbojet engine. The results of the engine test showed an essentially increased service life and form stability of the flameholder guide vane. Example 3 A ceramic insert of the shape according to Fig. 3 was manufactured in a ceramic composite material. The material was C-fibre/Si 3 N 4 ~matrix manufactured by a hot isostatic pressing step (HIP) by ABB Cerama AB, Sweden. The insert was placed within a flameholder guide vane such as illustrated in Fig. 3. The guide vane was welded and soldered in the same way as when manufactured without ceramic insert. The metallic casing was entirely without openings, i.e. non-cooled. Also the ceramic insert was free from holes or openings. The flameholder was mounted in the after-burner portion of a turbojet engine. The results of the engine test showed an essentially increased service life and form stability of the flame holder guide vane. Example 4 A ceramic insert in the shape of only the side wall of a guide vane according to Fig. 3 was manufactured in a ceramic composite material. The material was SiC-fibre/Al 2 0 3 - matrix manufactured with the Dimox®-method by DuPont Lanxide Composites, Inc., USA. The centre volume was filled with a fibre isolating block of aluminium silicate fibre with mul¬ lite composition called •-Fiberfrax® 11 , Carborundum, USA. The insert was placed within a flame holder guide vane 4 such as illustrated in Fig. 3 of the drawing. The guide vane 4 was welded and soldered in the same way as when manufactured without ceramic insert. The metallic casing was free from holes, i.e. non-cooled. The flame holder was mounted in the after-burner of a turbo-jet engine. The results of the engine test showed an essentially increased service life and form stability of the flameholder guide vane.

Claims: C l a i m 1. A flameholder device, particularly for ram-jet engi- nes and after-burners of turbojet engines and comprising an annular body (2) coaxial with the centre axis of the engine and made as a plurality of circumferentially spaced thin- walled metal guide vanes (4) , upstream of which fuel nozzles (1) are mounted and downstream of which a turbulent combus- tion-stabilizing zone is formed, said guide vanes (4) being hollow and having relatively great volume and which, if desired, are cooled by means of air flowing therethrough, the guide vanes furthermore being protected by a ceramic material (3), c h a r a c t e r i z e d i n that the ceramic mate- rial is made as a closed box-like insert body (3) arranged within the cavity of the guide vanes and with its external surface all over being located close to the internal surface of the guide vane walls and which either is hollow in itself or, in the case with cooling by means of an air flow, has openings corresponding to the through-flow openings in the guide vane walls, or is filled with a porous ceramic material or entirely consists of such porous material.

Inventors: IVARSSON PETER, LUNDBERG ROBERT, IVARSSON, PETER, LUNDBERG, ROBERT
Applicants: IVARSSON PETER, LUNDBERG ROBERT, VOLVO FLYGMOTOR AB, IVARSSON, PETER, LUNDBERG, ROBERT
Application year: 1994
Main classification: F23R3
IPC codes: F02K3, F02K7, F23R3